One category of well pump is an electrically driven rotary pump that is driven by a downhole electrical motor. These types of pumps operate best when pumping fluid that is primarily liquid. If the well fluid contains large quantities of gas, a gas separator can be connected to the pump assembly upstream of the pump for separating gas in the well fluid and discharging it into the casing. A common type of gas separator has rotatable vanes that separate the gas by centrifugal force.
While a gas separator works well enough to separate gas prior to the entry in the pump, another problem exists, particularly in horizontal wells where slugging is a problem. The term “gas slugging” refers to large gas bubbles that are encountered and which may require several minutes to dissipate through the pump or gas separator and into the casing. Normally, the motor of the pump is located below the pump and in a position so that well fluid flows over it for cooling the motor as the well fluid flows into the intake of the pump. If large gas bubbles are encountered, the motor could heat drastically during the interim that no liquid is flowing over it.
One solution is to place the motor within a shroud and locate the inlet of the shroud below the perforations. This requires the well fluid to flow downward from the perforations into the inlet of the shroud, then back up to the intake of the pump within the shroud. As the well fluid flows downward, some of the gas will separate from the well fluid and flow upward, reducing the amount of gas that flows into the shroud. While this works well enough in areas where a shroud intake can be placed below the perforations, in some cases, it is not possible to locate a shroud intake below the perforations.